1. Field of the Invention
The present invention relates to optical devices for providing decorative or entertaining visual effects, and in particular, to optical devices for projecting virtual images.
2. Description of Related Art
It is well known to produce in front of a lens a virtual image of a three dimensional object behind the lens. For example, in FIG. 2 of U.S. Pat. No. 5,257,130 light shining on an object 12 is reflected and transmitted through a double convex lens 27 to produce a virtual image 16 in front of a scrim 44. See also U.S. Pat. No. 6,594,083. Such virtual images have also been created with Fresnel lenses. In some cases a three dimensional object is rotated so that the virtual image rotates as well. See U.S. Pat. No. 4,261,657.
The foregoing arrangements rely on an external light shining on the surface of the object. The inventor has discovered that inappropriate external lighting of an object is a source of undesirable optical effects. In particular, a three dimensional object reflecting light from an external light source will project a virtual image that is surrounded by an aura. In general, a projected virtual image was found to be highly sensitive to the type of illumination and care must be taken to avoid the aura effect or other undesirable visual effects.
In FIGS. 1-3 of U.S. Pat. No. 3,293,983 an external light again shines on object 30 so that an aura will be created around virtual image 39. For the embodiment of FIG. 4 the display object 45 and its support 44 are both transparent (and presumably illuminated as before). Regardless of any aura effect, transparent display objects with transparent supports are undesirable because the support has the same visual prominence as the display object. Also, transparent objects tend to produce ghost-like images and often transmit “hot spots” originating from the background or from the illumination source. Hot spots can be especially problematical when an object is backlit as in FIG. 5.
In U.S. Pat. No. 3,868,501 hot spots will be extremely distracting in that a large lightbulb 41 is placed behind a transparent panel 43 that is imprinted with a design 42. The resulting virtual image in front of Fresnel lens 41 is shown as a lightbulb bearing the image of transparent panel 43.
In U.S. Pat. No. 6,375,326 and U.S. Patent Application Publication No. 2002/0012105 an image from source 10 may be transmitted through beam splitter 13 and Fresnel lens 11 before being reflected by mirror 12 and sent back again through Fresnel lens 11; finally being reflected outwardly by beam splitter 13. This reference does not describe how illumination is handled in image source 10.
In FIG. 11 of U.S. Pat. No. 4,571,041 reflected light from an externally illuminated object 116 is transmitted through lens 120, reflected by reflector 126 and then transmitted through lens 118 to produce a virtual image 130 in front of the lenses.
See also U.S. Patent Application Publication No. 2002/0126396.
Materials have been categorized as transparent, translucent, or opaque. Opaque materials transmit essentially no light, while transparent materials transmit a high percentage of incident light while maintaining image clarity; i.e., one can clearly see objects on the opposite side of transparent materials. Translucent material will transmit light but will not maintain image clarity, so someone cannot easily see objects on the opposite side of translucent material. A great number of physical phenomena can cause image degradation in a translucent material. Scattering or diffusion of light can be caused by interaction of light with the translucent material at an atomic or molecular level. Also, macroscopic, microscopic, or colloidal particles in a translucent material can also diffuse light. For situations where light diffusion occurs throughout a volume through which light travels, the diffusion of the light can be characterized by a scattering coefficient. On the other hand, some materials may have a roughened surface (e.g., etched glass or roughened plastic) that diffuses light through a complex combination of refraction, reflection, interference, etc. and is not easily characterized by a scattering coefficient.
Fundamentally, light transmitted through translucent material is mostly non-specular, meaning the emerging light is spread over an angular distribution, even when the incident radiation is a coherent or collimated beam arriving at a discrete angle. Nevertheless, some translucent materials will have both specular and diffuse transmission. In such cases the transmitted light has been characterized by a haze parameter defined as the ratio between the diffuse part of the transmitted light to the total transmitted light. Instruments for measuring the haze parameter are offered by Byk-Gardner of Geretsried, Germany.
Ideal light diffusion will have a Lambertian distribution, in which the light intensity varies with emission angle as a cosine function. Since the effective area of a Lambertian source also decreases as a cosine function of the emission angle in the same proportion as the intensity, the brightness of the Lambertian surface is constant for all emission angles (i.e., uniform brightness for a solid angle of substantially 2π steradians). Opal glass is an example of a Lambertian diffuser, but one with low efficiency. Not all translucent material will diffuse light with a Lambertian distribution and in some cases the non-specular light will have a Gaussian distribution, or some other angular distribution.